of air into the tire,
which thus in time becomes inflated. In most cheap bicycle pumps, the
piston valve is replaced by a soft piece of leather so attached to the
piston that it allows air to slip around it and into the cylinder, but
prevents its escape from the cylinder (Fig. 144).

[Illustration: FIG. 144.--The bicycle foot pump.]

191. How a Man works under Water. Place one end of a piece of glass
tube in a vessel of water and notice that the water rises in the tube
(Fig. 145). Blow into the tube and see whether you can force the water
wholly or partially down the tube. If the tube is connected to a small
compression pump, sufficient air can be sent into the tube to cause
the water to sink and to keep the tube permanently clear of water.
This is, in brief, the principle employed for work under water. A
compression pump forces air through a tube into the chamber in which
men are to work (Fig. 146). The air thus furnished from above supplies
the workmen with oxygen, and by its pressure prevents water from
entering the chamber. When the task has been completed, the chamber is
raised and later lowered to a new position.

[Illustration: FIG. 145.--Water does not enter the tube as long as we
blow into it.]

Figure 147 shows men at work on a bridge foundation. Workmen, tools,
and supplies are lowered in baskets through a central tube _BC_
provided with an air chamber _L_, having air-tight gates at _A_ and
_A'_. The gate _A_ is opened and workmen enter the air chamber. The
gate _A_ is then closed and the gate _A'_ is opened slowly to give the
men time to get accustomed to the high pressure in _B_, and then the
men are lowered to the bottom. Excavated earth is removed in a similar
manner. Air is supplied through a tube _DD_. Such an arrangement for
work under water is called a caisson. It is held in position by a mass
of concrete _EE_.

[Illustration: FIG. 146--The principle of work under water.]

[Illustration: FIG. 147--Showing how men can work under water.]

In many cases men work in diving suits rather than in caissons; these
suits are made of rubber except for the head piece, which is of metal
provided with transparent eyepieces. Air is supplied through a
flexible tube by a compression pump. The diver sometimes carries on
his back a tank of compressed air, from which the air escapes through
a tube to the space between the body and the suit. When the air has
become foul, the diver opens a valve in his suit and al